A typical gas turbine engine combustor includes a generally annular chamber having a plurality of fuel injectors at an upstream head end. Combustion air is provided through the head and in addition through primary and intermediate mixing ports provided in the combustor walls, downstream of the fuel injectors.
In order to improve the thrust and fuel consumption of gas turbine engines, i.e. the thermal efficiency, it is necessary to use high compressor pressures and combustion temperatures. Higher compressor pressures give rise to higher compressor outlet temperatures and higher pressures in the combustion chamber, which result in the combustor chamber experiencing much higher temperatures than are present in most conventional prior combustor designs.
There is therefore a need to provide effective cooling of the combustion chamber walls. Various cooling methods have been proposed including the provision of a doubled walled combustion chamber whereby cooling air is directed into a gap between spaced outer and inner walls, thus cooling the inner wall. This air is then exhausted into the combustion chamber through apertures in the inner wall. The inner wall may comprise a number of heat resistant tiles, such a construction being relatively simple and inexpensive.
Combustion chamber walls which comprise two or more layers are advantageous in that they only require a relatively small flow of air to achieve adequate cooling. However they are prone to some problems. These include the formation of hot spots in certain areas of the combustion chamber wall. Prior art proposals to alleviate this problem include the provision of raised lands or pedestals on the cold side of the wall tiles, these lands or pedestals serve to increase the surface area of the wall element thus increasing the cooling effect of the air flow between the combustor walls. Compressor delivery air is convected between pedestals on the ‘cold face’ of the tile and emerges as a film directed along the ‘hot’ surface of the following downstream tile.
The provision of such lands is also accompanied by inherent problems. For example localised overheating may occur behind obstructions such as mixing ports or adjacent to regions of near stochiometric combustion conditions (hot streaks). A particularly hot region has been recently identified on the combustor wall immediately downstream of the fuel injectors. There is no provision for enhanced heat removal, either locally to remove hot spots or to alleviate more general overheating towards the downstream end of the tile. Overheating may occur downstream of the mixing ports since the protective wall cooling film is stripped away by the transverse mixing jets. Where design requirements have dictated a relatively long tile the cooling film quality towards the downstream edge of the tile may be poor and may lead to local overheating.
To alleviate the above problems, it is known to provide a low conductivity thermal barrier coating on the hot side of the tiles and/or to provide effusion holes within the tiles, to effect localised cooling. Such effusion holes are preferably angled, as this provides an increased cooling surface, and helps to lay down a cooling film on the hot side of the tile. The effusion holes are typically formed by laser drilling.